Magneto-strictive resonators



Filed June 21, 1955 w ELL Lx: A

United States Patent O MAGNETO-STRICTIVE RESONATORS William Oliver Agar,Danbury, England, assigner to Marconis Wireless Telegraph CompanyLimited, Loudon, England, a British company Application June 2'1, 1955,Serial No. 516,813

kClaims priority, application Great Britain June 23, 1954 s Claims. (ci.S33- 71) This .invention relates to magneto-str-ictive resonators andmore specifically to frequency selective resonant electrical filters ofthe kind employing magneto-strictive mechanical resonators. Theinvention has for its object to provide improved filters `of this kindand which shall be of relatively simpleand economical construction, andof high Q value, i.e. of low damping.

According to the present invention there is provided a resonant filterof the kind referred to having a hollow cylindrical mechanical resonatormade of a magnetostrictive ferromagnetic ceramic material which ismagnetized with lines of force running round the annulus whichconstitutes the cross section of the resonator, a support wire for saidresonator, the wire running axially of the resonator, means comprisingan insulating member whose mechanical hardness is of the same order asthat of the material of said resonator, said means being attached to thesupport wire and to the resonator so as to support the latter centrallyof the length thereof, and at least one coil arranged .to excite saidresonator to oscillate in torsion.

Preferably the insulating member is a borate bead fused to the supportwire and to `the inside of the resonator.

Preferably the borate bead is keyed `to the support wire by thickening`the latter where the bead is to be positioned, or by providing the wirein the said position with a few turns of fine copper or other wirethereon or with a short length of sleeve thereon.

For a better understanding of the invention and to show how the same maybe carried into effect reference will now be made to the accompanyingdrawing in which:

Fig. 1 is a sectional elevation of a resonator, secured to a supportwire by a first fixing arrangement, and

Fig. 2 `shows the resonator of Fig. 1, when secured to the support wireby a different fixing arrangement.

Referring to Fig. l of the drawing the mechanical resonator portion -ofthe device therein `shown consists of a hollow elongated cylinder 1 offerromagnetic magnetostrictive ceramic material, for example thematerial commercially known as ferroxcube grade B. The length of thecylinder -is substantially equivalent to a half wave length at thedesired resonant frequency which, to quote a practical example, might bebetween say 80 kc./s. and 120 kc./s. The cylinder 1 is supported from asupport wire 2 which passes axially therethrough and may conveniently bemade of the material known as Nichrome. The cylinder is carried from thewire by means of a borate bead '3 which is fused on to the wire 2 andalso to the inside of the cylinder 1 as nearly as possible midway alongthe same. The bead 3 is of `the same order of hardness as thecylinder 1. The fusing `is effected to give firm contact without slipbetween the bead 3 and the wire 2 and especially between the bead 3 andthe cylinder 1. The area of contact between bead 3 and cylinder 1 ismade as small as is compatible with 2,895,113 'Patented July 14, 1959good mechanical strength. In order to give a firm keying of the bead onto the wire 2 a few turns of copper wire 4 are first wound onto the saidwire 2. This copper wire 4 in effect provides a key for the borate bead2. Instead of fixing the cylinder to the support wire by the arrangementshown in Fig. l, a sleeve may be provided on the support wire as shownin Fig. 2, where the sleeve is indicated at 4A. In Fig. 2 like parts arereferred to in Fig. 1 and are indicated by like numbers. An alternativemeans for xing the resonator to the supportwire is to locate a-thickened portion of the sup- .port wire midway along the resonatorinstead of using the sleeve 4A or fixing wire 4. Outwardly of the endsof the cylinder 1 are two plugs 5 and 6 also of ferroxcube material butof non-magneto-strictive quality, e.g., grade A material. These plugs 5.and 6 are centrally bored to pass the wire 2. They serve to reduce straymagnetic fields and, therefore, undesired coupling between one filterand another, similar adjacent one. They also serve .to ensuresubstantially constant coupling conditions between the coils (not yetreferred to) and the mechanical resonators of filters of the samegeneral design but different operating frequencies. Such lters ofdifferent operating frequencies will, of course, have cylinders ofdifferent lengths. By providing ferro-magnetic plugs 5 and 6 at the endsof the cylinders a series of devices of the same general design butdifferent resonant frequencies may be made with uniform couplingconditions throughout as between the mechanical resonators and theassociated coils.

Protection for the ferroxcube material against dirt, grease and .thelike is provided by a glass tube 7 and glass end sealing ybeads 8 and 9as shown, the plugs r, 5 and 6 and the beads 8 and 9 being fixed inposition by any suitable cement securing them to the tube 7. The plugs 5and 6 also serve to hold support wire 2 in position.

The `device shown has two coils, an input coil and an output coil, in acoil circuit 10 on the glass cylinder midway along the length of theferroxcube cylinder 1. The input or primary coil will normally be ofrelatively few turns wound at right angles to the axis of the resonatorand positioned midway along the length of the resonator and serves forenergizing the mechanical resonator into torsional vibration. Thesecondary or output coil will generally 'have considerably more turnsand is for the purpose of coupling the vibrating resonator to theutilization circuit though, of cou-rse, the number of turns in primaryand secondary will depend upon design requirements. The connections ltothe primary are represented fat 11 and those to the secondary at 12.

The mechanical resonator 1 vibrates in a torsional mode because of theresultant helical magnetic eld produced by the cylindrical permanentmagnetism of the resonator combined with the axial magnetic fieldproduced by the energized primary winding. Magnetization is easilyeffected in the magneto-stricture cylinder by passing a short pulse ofcurrent through the support wire 2.

It has been found in practice that with a device as illustrated anddesigned for a frequency of 'the order of to 120 kc./s., a Q value ofthe order of 4000 to 6000 was readily obtainable while the device itselfwas small, compact, relatively )simple to manufacture and relativelyeconomical. A series of devices as illustrated but resonant .atdifferent adjacent frequencies provides a most convenient and compactfrequency spectrum analyzer costing substantially less than, occupyingless space than, and giving a sharpness of frequency selectively as goodor better than those obtainable with known apparatus.

While I have `described my invention in certain of its preferredembodiments, I realize that modifications may be made, and I desiretha-t it be understood that no limitations upon my invention areintended other than may be imposed by the scope `of the appended claims,

I claim:

1. A frequency selective resonant electrical filter com* prising ahollow cylindrical mechanical resonator made of a magneto-strictiveferromagnetic ceramic material which is permanently magnetized andhaving lines of force running round the annulus which constitutes thecross section of the resonator, a support wire for said resonator, thewire running axially through the resonator, means for supporting saidwire adjacent its ends, means comprising an insulating member whosemechanical hardness is of the same order as that of the material of saidresonator, said means being attached to the support wire and to theresonator so as to support the latter centrally `of the length thereof,a rst coil wound axially round the resonator and positioned midway alongthe length of the resonator whereby when the coil is energized byalternating current it produces an axial alternating magnetic field andthe resonator is excited to vibrate in torsion and a second coilarranged near to said resonator whereby said second coil is energized bythe vibrating resonator.

2. A frequency selective resonant electrical lter as set forth in claim1, wherein the insulating member is a borate bead.

3. A frequency selective resonant electrical lter as `set forth in claim1, wherein the insulating member is a borate bead fused to the supportwire and to the inside of the resonator.

4. A frequency selective resonant electrical lter as set forth in claiml, wherein the insulating member is a borate bead keyed to the supportwire by thickening the latter at the location where the bead is to bepositioned.

5. A frequency selective resonant electrical filter as set forth inclaim 1, wherein the insulating member is a borate bead keyed to thesupport wire by providing around the latter a number of turns of wire atthe lo- 5 cation where the bead `is to be positioned.

6. A frequency selective resonant electrical filter as set forth inclaim 1, wherein the insulating member is a borate bead keyed to thesupport wire by providing a sleeve on the support wire at a locationwhere it is desired to position the bead.

7. A frequency selective resonant electrical filter as set forth inclaim 1, wherein means are provided adjacent each end of the resonatorfor minimizing the eect of stray magnetic elds upon the oscillations ofthe res-- l5 onator, said last mentioned means being displaced away fromthe magnetic elds produced by said coils.

3. A frequency selective resonant electrical filter as set forth inclaim 1, wherein means are provided adjacent each end of the resonatorfor minimizing the eiect of stray magnetic iields upon the oscillationsof .the resonator, said last mentioned means being located away from themagnetic field provided by said coils and including plugs bored to passthe support wire, the plugs being formed from a non-magneto-strictivematerial.

p 20 References Cited in the le of this patent UNITED STATES PATENTS1,966,446 Hayes July 17, 1934 1,997,599 Pierce Apr. 16, 1935 2,435,487Adler Feb. 3, 194s 2,472,388 Thuras June 7, 1949 2,692,344 Van Der Burgtet al Oct. 19, 1954 2,709,243 Babcock May 24, 1955 2,736,824 RobertsFeb. 28, 1956 2,770,782 Roberts Nov. 13, 1956

